Drainage Design Manual, Hydrology - Flood Control District of ...

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Drainage Design Manual for Maricopa County Hydrology: Application 4. If the watershed or subbasin is composed of soils of different textures, then areaweighted parameter values will be calculated: a. Calculate the area-weighted value of XKSAT by using Equation (4.4). b. Select the corresponding values of PSIF and DTHETA from Figure 4.3. c. Calculate the arithmetically area-weighted value of naturally occurring RTIMP. 5. Select values of IA for each land use and/or soil cover using Table 4.2. Arithmetically area-weight the values of IA if the drainage area or subbasin is composed of subareas of different IA. 6. Select values of RTIMP for each land use using Table 4.2. Arithmetically area-weight the values of RTIMP if the drainage area or subbasin is composed of land use subareas of different RTIMP. Compute the weighted value of RTIMP based on the areaweighted land use and denote it by RTIMP L . Arithmetically area-weight the rock outcrop percentages for all soil map units to obtain RTIMP N . Estimate the effective percentage of rock outcrop for each soil map unit that is hydraulically connected. Arithmetically area-weight the effective percentage of rock outcrop for all soil map units to obtain EFF. Compute the final composite value of RTIMP using Equation (4.6). RTIMP = RTIMP L + EFF (RTIMP N ) 7. Estimate the vegetative cover (VCD) for the natural portions of the drainage area or subbasin. Select values of VC for each land use using Table 4.2. Arithmetically areaweight the values of VCD if the drainage area or subbasin is composed of land use subareas of different VCD. Arithmetically average the natural VCD and the areaweighted land use VCD. 8. Adjust the XKSAT value for VC using Figure 4.4, if appropriate. 9. Arithmetically average DTHETA dry (natural portions of the drainage area or subbasin) and DTHETA normal (Developed portions of the drainage area or subbasin), if appropriate. B. Alternative Methods: As an alternative to the above procedures, Green and Ampt loss rate parameters can be estimated by reconstitution of recorded rainfall-runoff events on the drainage area or hydrologically similar watersheds, or parameters can be estimated by use of rainfall simulators in field experiments. Plans and procedures for estimating Green and Ampt loss rate parameters by 9-26 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Application either of these procedures should be approved by the Flood Control District and/or the local agency before initiating the procedures. 9.3.2 Procedures for the Initial Loss Plus Uniform Loss Rate Method A. When soils data are available: 1. Prepare a base map of the drainage area delineating modeling subbasins, if used. 2. Delineate subareas of different infiltration rates (uniform loss rates) on the base map. Assign a land-use or surface cover to each subarea. 3. Determine the size of each subbasin and size of each subarea within each subbasin. 4. Estimate the impervious area (RTIMP) for the drainage area or each subarea. 5. Estimate the initial loss (STRTL) for the drainage area or each subarea by regional studies or calibration. Alternatively, Equation (4.5) or Table 4.2 and Table 4.4 can be used to estimate or to check the value of STRTL. 6. Estimate the uniform loss rate (CNSTL) for the drainage area or each subarea by regional studies or calibration. Table 4.3 can be used, in certain situations, to estimate or to check the values of CNSTL. 7. Calculate the area-weighted values of RTIMP, STRTL, and CNSTL for the drainage area or each subbasin. 8. Enter the area-weighted values of RTIMP, STRTL, and CNSTL for the drainage area or each subbasin on the LU record of the HEC-1 input file. 9.3.3 User Notes 1. There are currently six soil survey volumes available for Maricopa County and adjoining areas. Five of these are published by the National Resource Conservation Service (NRCS). A figure showing the status and extent of each NRCS survey is provided at the front of APPENDIX C. Copies of these survey reports can be obtained from the NRCS field offices. Data from three of these surveys have been summarized and are included in APPENDIX C, Appendix C.2, Appendix C.3 and Appendix C.4 along with map unit values of XKSAT and rock outcrop percentages. The sixth soil survey is published by the Forest Service and is entitled Tonto National Forest Terrestrial Ecosystem Survey. A copy of this survey can also be obtained from the Forest Service field office. August 15, 2013 9-27
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